We present the results of X-ray observations of the broad-line radio galaxy 3C 390.3 obtained with ASCA. The 0.5-10 keV spectrum of the source is well described by a power-law model plus a component arising from Compton reflection and an emission line. There is no evidence for a soft excess. The photon index is 1.70 +/- 0.04 and the absorbing column density is (8.8 +/- 1.5) x 10(20) cm(-2), which is somewhat larger than the neutral hydrogen column density in our Galaxy. An iron K alpha line is detected unambiguously, with an equivalent width of 190(-70)(+100) eV (90% confidence), and resolved. It has a centroid energy of 6.34(-0.10)(+0.17) keV in the rest frame of the source and hence corresponds to fluorescent emission from ''cold'' iron. The full width of the line at half-maximum corresponds to a velocity between 7000 and 43,000 km s(-1) (90% confidence). If the line width is attributed to Keplerian rotation in an accretion disk with an inclination angle of 26 degrees (inferred independently from model fits to the double-peaked H alpha line and from superluminal motions in the compact radio jet), then its measured equivalent width and centroid energy are in agreement with models for the reprocessing of X-rays from cool, dense material in the disk, at about 250 gravitational radii from the center. An alternative origin of the Fe K alpha line in the hot, innermost part of the disk is disfavored on the basis of the small velocity width and the centroid energy, which is consistent with ''cold'' iron. An origin in standard broad-line region clouds is also unlikely because of the large equivalent width. However, an origin of the line in Thompson-thick clouds is not ruled out by the data. The absence of absorption edges from highly ionized metals, and the low-ionization stage of iron inferred from the properties of the K alpha line suggest that the ionization parameter in the line-emitting region is less than 100.
